The invention relates to a Luneberg lens having a refractive index profile changing from the center of the lens towards its perimeter.
Integrated optics is an attractive approach for providing new devices for signal processing such as scanners, deflectors, modulators, switches, RF spectrum analyzers, multiplexers and demultiplexers because of the potential for very high performance operation and high speed processing using optical processing principles in structures which can be very small and rigid in planar configuration, and which can be manufactured by batch fabrication techniques. Such devices need thin-film waveguide lenses to control the shape of the guided beam for imaging, spatial filtering, focusing and Fourier analysis. The lenses for these applications must have high efficiencies, high performance and high stability. Further, lens accuracy, i.e. the focal length of the lens shape being accurate enough to satisfy design specifications, is essential for more precise applications such as where a well-collimated guided beam or sufficiently small beam spot size is needed.
A conventional lens for optical imaging of electromagnetic waves is based on a physical structure (concave or convex lens etc.) in which the dielectric constant encountered by a light wave changes abruptly at the air/lens interface. Focusing and other effects are based on the angle of incidence of the light wave and the contour of the lens surface. Special care must be taken to avoid effects such as back-reflection of diffraction or aberrations (e.g. due to off-axis effects).
Focusing can also be established by refraction of light based on a gradually changing index of refraction within an otherwise uniform material. In this case there is no abrupt change between two materials but rather a gradient in the index structures.
Graded-index (GRIN) lenses are useful for optical communications and in imaging systems because they do not rely on shape for their optical properties. A completely flat piece of radial-GRIN material can act as a lens. There is no abrupt physical interface between two materials with different indices of refraction but rather a gradient in the index of refraction. For example, an optical fiber is based on core and cladding materials with different indices of refraction. Although the transition between core and cladding can be abrupt, a smoothly varying transition provides a variety of advantages. In this case, the index gradient is radial, normal to the direction of the fiber. More commonly seen are rod structured GRIN-lenses in which an index gradient establishes the refractive properties of the lens. These can be reduced to small size and readily abutted against a fiber or other optical interfaces. Since they can be manufactured with precise optical and physical structures, GRIN lenses are widely used in miniaturized optical and optoelectronic systems.
Despite the usefulness of the devices, there are relatively few practical ways of manufacturing. Plastic fibers, which have been irradiated to change their optical properties radially, tend to be of low optical quality. Sol-gel processes do not allow large refractive index variations. The chemical-vapor-deposition (CVD) method, in which a hollow glass tube is filled with a glass-producing vapor that varies over time, works well but is expensive. The most mature manufacturing method today employs ion-exchange techniques, which involve ions diffusing into the surface of a glass rod.
Conventional lenses as well as GRIN Rod lenses suffer from back-reflection due to the index mismatch between lens material and the ambient material (i.e. air). Due to the index profile incorporated into Luneberg lenses they provide index matching to their environment and thus do not suffer from such reflection losses.
A Luneberg lens is a spherical lens without a specific direction serving as an optical axis. Each incident beam is directed towards the center of the sphere and is incident along an optical axis. Therefore, the Luneberg lens has performance advantages, specifically related to the aberrations due to off-axis effects in conventional lenses.
Luneberg lenses can be produced at low cost because an appropriately made fabrication mask is repeatedly useable. In addition, fabrication is easy because it involves a conventional deposition apparatus, and the thickness of the film is highly controllable. It is very important to control the lens shape since the lens characteristics are very sensitive to the film thickness and index distribution. Finally, diffraction-limited lenses with accurately predictable focal length can be routinely made. Still, for some applications the control of the non-homogeneous refractive index in the lens material obtainable by p.ex. diffusion is not accurate enough.
U.S. Pat. No. 4,403,825 discloses a method for lens formation employing illumination of a photosensitive film through an aperture with adjustable size to produce the required effective index profile. However, this method does not allow sufficient control of the index profile within the photosensitive layer. In particular, a chalcogenide glass layer is suggested as photosensitive layer. This has the disadvantage that its optical characteristics are not stable due to the adverse effects of light and/or moisture. Furthermore, some applications require much more accurate focal length and focal spot sizes as is possible with the lenses according to the state of the art.